Distributed Generation (DG) systems are commonly fed by renewable energy sources such as wind or PV (photo-voltaic) technology. In all grid-connected DG systems, a conversion between DC and AC power is performed by an inverter system. In the case of wind energy, AC power is generated by a rotating turbine which is rectified to DC power by a front-end converter as depicted by FIG. 1. PV energy, or more commonly referred to as solar power, is generated by light contacting an array of photo-sensitive diodes which develops a DC voltage across the device and current through the device upon circuit completion. It can be seen in FIG. 2 that the developed power undergoes a DC/DC conversion by the front-end converter to meet the DC power requirements of the second stage converter. In both cases of wind and PV generators, the front-end converter is responsible for the extraction of maximum-power from the volatile energy sources. The grid-connected converter in both systems is responsible for the conversion of DC power into AC power which meets the interconnection quality standards of the region.
The inverter is the interface between any renewable energy source and the electricity grid. The inverter is composed of many different systems which are designed to ensure that the quality of the electric current leaving the inverter is compliant with the interconnection standard (e.g., IEEE1547). The conventional grid-connected DC/AC system is composed of a synchronizer, a current controller, and a DC-bus regulator. The interconnection of these systems is shown in FIG. 3. The synchronizer generates the sinusoidal current reference which is required to be free of distortions and to be phase-locked to the electricity grid voltage phase. The current controller assures that the current injected into the grid is in-phase with the reference signal and that harmonic content is at a minimum. The DC-bus regulator adjusts the magnitude of the current reference signal in order to maintain the voltage in a safe operating region.
The critical component to the DC/AC converter is the current control loop responsible for the fast and accurate tracking of the reference signal. A sinusoidal signal is not easily tracked by linear systems particularly when higher order output filters are implemented. Such filters also tend to be susceptible to resonant excitation and require special attention to prevent such instabilities. Typically, a state-feedback loop is implemented to promote stability. Another requirement of the current controller is that it should reject harmonic content which occurs due to switching device non-linearities or unintentional reference noise.
The basic requirement for a current controller is the ability to track a sinusoidal reference. The introduction of LCL-filters, a third-order system with a resonance peak, poses challenges in terms of closed-loop control system stability. The LCL-filter is described as a linear forced oscillation system: the controller is forcing the system to resonate at the grid frequency but the system will always tend to resonate at its natural frequency. FIG. 4 shows the grid connected DC/AC inverter with an LCL-filter.
FIG. 5 shows the block diagram of the closed-loop current control system for a grid-connected DC/AC inverter with an LCL-filter according to the prior art. According to FIG. 5, the current control loop includes Proportional Resonant (PR)-controllers along with a linear state-feedback controller. The resonant controllers are responsible for the sinusoidal reference tracking of the grid current as well as for eliminating the current harmonics by providing very high loop gains at the harmonic frequencies. The linear state-feedback guarantees the stability of the closed-loop control system by placing the closed-loop poles on the left-half plane.
The main difficulty with the current controller shown in FIG. 5 is that the closed-loop system is only stable for nominal values of the system parameters. If the parameters vary, the stability of the current control loop might be jeopardised. The state-feedback gains are designed based on the values of the components of the LCL-filter and these values are subject to change based on operating conditions and grid impedance. In addition, if the grid frequency changes, the PR-controller is unable to perform perfect tracking of the sinusoidal reference signal. As distributed power generators become more prevalent and conventional energy sources are replaced with renewable energy sources, grid conditions become more volatile and grid impedance can change significantly. Because of this, the current controller in grid-connected inverters should be able to maintain stability despite variations in grid conditions.